I. FIELD OF INVENTION
This invention relates to an improved method of isolating and recovering 2,4-dinitro-N-substituted-1,3-phenylenediamine compounds having a trifluoromethyl substituent in the 6-position of the aromatic ring from the reaction medium.
II. DESCRIPTION OF THE PRIOR ART
U.S. Pat. Nos. 3,617,252 and 3,764,623 describe methods for making and using the above class of herbicidal compounds. These patents in their entirety are incorporated herein by reference. The above compounds are defined by the formula: ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each selected from hydrogen, lower alkyl, lower alkenyl, lower alkynyl, cyclo lower alkyl and cyclo lower alkenyl groups, said groups other than hydrogen being unsubstituted or having chloro, bromo, iodo, hydroxy or lower alkoxy substituents, with the proviso that at least one of R.sub.1, R.sub.2, R.sub.3 or R.sub.4 is not hydrogen. The lower alkyl, lower alkenyl, lower alkynyl and lower alkyoxy groups as used above are defined generally to have up to about six carbon atoms.
According to the cited prior art, the compounds of formula I are prepared generally by reacting a 3-halo-2,6-dinitro-4-trifluoromethylaniline compound with a stoichiometric excess of either ammonia, a primary amine or a secondary amine in the presence of an appropriate polar organic solvent (see equation A below).
While some members of this class of compounds have shown outstanding herbicidal properties, their widespread commercial acceptance has been somewhat hindered by troublesome processing conditions. This is especially true of the final isolation and recovery steps for making these herbicidal compounds. For instance, the above-cited patents only disclose recovering these compounds by first either water-washing or filtering techniques to remove by-product salts, followed by solvent evaporation and recrystallization steps. While these procedures may be suitable recovery methods for laboratory-scale experiments, they are often impractical on a layer commercial level. In particular, the commercial employment of a filtration step would require a great deal of solids handling and a follow-up step of vacuum drying the filtrate, both of which are costly, time-consuming and energy-intensive. Additionally, removal of the polar organic solvent by simple evaporation is time-consuming. Furthermore, it has been found, according to this invention, that the heat used in an evaporation step may have the effect of decomposing the product, thereby reducing the over-all yield.
Therefore, a need still exists in the art to improve the steps of isolating and recovering these compounds so that more attractive (i.e., less costly and higher yield) commercial products can be made. Such an improvement should also result in a lowering of the energy requirements and simplify the processing steps.